Present energy costs have forced utilities to take a new look at the way in which they evaluate transformer losses. Braunstein, "The Way You Buy Transformers Affects the Price", Electrical World, July 1982, pg. 123. The evaluation of bids for transformers is an involved study encompassing many facets of engineering economics. When all factors are evaluated and tabulated, it is common to find that the lowest-priced item will cost more than the others in the long run. It is common, for example, to find that transformer losses will cost more over the life of the transformer than the original price of the transformer. Chartier, "The Economics of Major Equipment Evaluation", The Line 74-2 (1974) page 20. Thus, transformer losses frequently become the most significant factor in the buying decision. A change in a few percent can spell the difference between a successful bid and a rejected bid.
The transformer industry in the United States is highly developed. There have been few major breakthroughs over the last ten years or so. This does not mean that there is no need for improvement or that further improvement cannot be made. However mature and sophisticated the design of transformers may be, the problem of finding an optimum design is far from obvious. The engineer is often faced with conflicting alternatives and limited choices.
The design of successful commercial transformers requires the selection of a simple structure or form, so that the conductor coils and insulation are easy to wind and the magnetic (iron) circuit is easy to build. At the same time, the mean length of the coil windings and the magnetic circuit must be as short as possible for a given cross sectional area so that the amount of material required and the losses are kept as low as possible. It is also desirable to operate at the highest flux density consistant with low losses in order to reduce the amount of iron and conductor. Two basic designs have emerged from these considerations.
When the magnetic circuit takes the form of a single ring encircled by two or more groups of primary and secondary windings distributed around the periphery of the ring, the transformer is termed a core-type transformer. When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic material distributed around its periphery, the transformer is termed a shell-type transformer. One characteristic feature of the shell-type transformer is the short mean length of the magnetic circuit and the relatively long mean length of the windings. Because of these differences in shape and form, design improvements to one are not necessarily adaptable to the other.
As another example of the difficulty facing the transformer engineer, consider what might be done to reduce the iron loss (no-load loss). The simplest solution is to reduce the voltage and leave the physical design the same. Since the iron loss will decrease approximately as the square of the voltage, one needs only to reduce the voltage by 5% to get a 10% reduction in loss. However practical this solution may be at first sight, one must realize that a 5% reduction in voltage is accompanied by a 5% loss in the effective transformer capacity. Furthermore, the load loss in percent of this reduced rating has increased by 5%! Thus, one ends up sacrificing transformer capacity and increasing the per unit load loss by about half the percentage of reduction in the no-load loss.
It is conventional wisdom that iron loss will vary with the weight of the iron. Thus, another approach to lower no-load loss is to change the physical design of the transformer to reduce the cross-section of the iron core, while increasing the number of conductor turns in the coil to keep the flux density constant. If this is done, the core window opening will have to be increased to accommodate the higher number of turns, but there may still be a substantial decrease in the weight of the core and the consequent no-load loss. The conductor loss (or load loss), however, will increase with the number of turns. For example, if one reduces the cross-section of the core by 5% and increases the turns in the coil by 5%, one can expect to obtain about a 5% reduction in core loss, but at the cost of an increase of about 5% in the load loss. Moreover, the reactance will increase nearly in proportion to the square of the turns, or in this case by about 10%. Therefore, reducing the core section and increasing the turns have the effect of: reducing the no-load loss by the percentage amount that the load loss increases; increasing the total loss (if the load loss was originally greater than the no-load loss); increasing the reactance; and decreasing the weight of iron by approximately the same percentage amount that the weight of conductor increases. Simply stated, low reactance does not necessarily go with high load loss or low iron loss; similarly high reactance does not necessarily go with low load loss. In fact, it is generally considered to be unreasonably expensive to try to design for a low iron loss and low reactance in the same transformer. Yet, low reactance is a real advantage in a distribution transformer, because it is necessary to have the lowest possible regulation in these transformers. One author concludes that: If low iron loss is important, it will be more economical to use as small a transformer as possible, and to load it (high load loss) as heavily as possible using forced cooling; and if load loss is important it may be more economical to simply use a larger transformer (high no-load loss). Bean, Transformers for the Electric Power Industry, McGraw-Hill Book Company, Inc. (1959)
However good this recommendation may be, the typical distribution transformer (particularly the pole-mounted transformer in the 5-to-167-KVA range) is lightly loaded for an appreciable portion of the 24-hour day. Because of this, the loss in the core is a significant portion of the total daily loss. Cores for these units are, therefore, designed for low exciting current (low reluctance) and for relatively low core loss to minimize the operating cost. Fink, Standard Handbook for Electrical Engineers, Eleventh Edition, Section 10, Paragraph 158. Thus, for distribution transformers, no-load losses are important and a design that lowers core losses by increasing the mass of iron in the core without increasing the reactance and load losses is by no means obvious. It should be equally clear that those principles which apply to large power transformers do not necessarily apply to small distribution transformers.
From the foregoing, it should be appreciated that the design of transformers, and distribution transformers in particular, still leaves room for improvement. An improved distribution transformer which would allow one to reduce the core losses by an amount in excess of what the gain would be in the size of the core, and without having a proportional effect on the reactance of the transformer and without increasing the load loss, would go far in achieving the ultimate in a distribution transformer. It would also have the advantage of satisfying the continued long-felt need, by utilities and other purchasers of distribution transformers, in reducing life-cycle costs. Finally, if this energy saving improvement could be easily and readily adapted to existing transformer designs, the improved transformer could be made available quickly to customers, and without developing special equipment or procedures, or extensive capital investment. Heretofore, no one distribution transformer design, particularly one of the wound core design, has been able to achieve these advantages and features in a simple construction.